In carrier sensing multiple access/collision avoidance (CSMA/CA) based wireless networks such as the Institute of Electrical and Electronics Engineers (IEEE) standard 802.11 networks, a transmitter relies on carrier sensing to determine if the air medium is available so as to avoid interference. With carrier sensing, a station samples the energy level in the air before starting a packet transmission, and a transmission is only started if the reading is below a threshold PCS, also called the physical carrier sensing (PCS) threshold. Because network characteristics such as topology, propagation, environment, background noise, etc., affect the energy level sample a station will measure, the choice of a PCS threshold that will enable CSMA to provide its best performance is not a trivial task. Moreover, these characteristics may change over time.
Unlike a wired LAN such as Ethernet, the transmission medium is not shared by the entire network in a wireless LAN. Rather, there exist multiple overlapping neighborhoods where the medium must be shared via contention. Moreover, modern wireless transceivers are designed to successfully receive packet transmissions, even in the presence of interference. The fundamental factor that determines whether a packet can be successfully received by a receiver is a signal to noise ratio (SNR), and specifically the signal to noise plus interference ratio (SNIR) at the receiver. If the signal that a device is attempting to receive has sufficiently more energy than the background noise and interference to be distinguishable over the background noise and interference, successful packet reception can occur even in the presence of interference. Thus, the goal of PCS in CSMA is to prevent simultaneous transmissions that will lead to packet collisions, while maximizing spatial reuse by permitting simultaneous transmissions that will not violate receiver SNIR requirements.
Current 802.11 networks typically operate with a physical carrier sensing scheme configured with a fixed threshold. The fixed threshold is typically very low, such that even a communication between network nodes spatially remote to the station in question would generate strong enough energy to make the station withhold its transmission. As a result, virtually no spatial reuse is allowed. Furthermore, the fixed threshold cannot be dynamically tuned according to different environments and as condition changes in the network. As wireless networks are deployed at higher densities and/or in multi-hop mesh topologies, the potential for spatial reuse increases. However, current PCS schemes with fixed threshold limit the ability to make full use of spatial capacity in these dense wireless network scenarios.
Other sensing schemes have been developed to improve system throughput. For example, virtual carrier sensing (VCS) schemes are also used in wireless networks. With VCS, a station maintains a NAV (Network Allocation Vector) that indicates the period(s) during which the air medium is reserved by other stations. This informs the station when NOT to transmit. When contending for the medium, a station broadcasts its intended transmission period. Each station that receives the broadcast updates its NAV. Thus, VCS requires participating stations to be able to receive and decode the broadcast frames. Unfortunately, this requirement (e.g. through RTS/CTS handshaking) cannot be guaranteed in most dense wireless networks including mesh networks.